This invention relates to ultrasonic devices and more particularly to devices for tuning and controlling an ophthalmic phacoemulsification handpiece.
A typical ultrasonic surgical device suitable for ophthalmic procedures consists of an ultrasonically driven handpiece, an attached hollow cutting tip, an irrigating sleeve and an electronic control console. The handpiece assembly is attached to the control console by an electric cable and flexible tubings. Through the electric cable, the console varies the power level transmitted by the handpiece to the attached cutting tip and the flexible tubings supply irrigation fluid to and draw aspiration fluid from the eye through the handpiece assembly.
The operative part of the handpiece is a centrally located, hollow resonating bar or horn directly attached to a set of piezoelectric crystals. The crystals supply the required ultrasonic vibration needed to drive both the horn and the attached cutting tip during phacoemulsification and are controlled by the console. The crystal/horn assembly is suspended within the hollow body or shell of the handpiece at its nodal points by relatively inflexible mountings. The handpiece body terminates in a reduced diameter portion or nosecone at the body's distal end. The nosecone is externally threaded to accept the irrigation sleeve. Likewise, the horn bore is internally threaded at its distal end to receive the external threads of the cutting tip. The irrigation sleeve also has an internally threaded bore that is screwed onto the external threads of the nosecone. The cutting tip is adjusted so that the tip projects only a predetermined amount past the open end of the irrigating sleeve. Ultrasonic handpieces and cutting tips are more fully described in U.S. Pat. Nos. 3,589,363; 4,223,676; 4,246,902; 4,493,694; 4,515,583; 4,589,415; 4,609,368; 4,869,715; and 4,922,902, the entire contents of which are incorporated herein by reference.
When used to perform phacoemulsification, the ends of the cutting tip and irrigating sleeve are inserted into a small incision of predetermined width in the cornea, sclera, or other location in the eye tissue in order to gain access to the anterior chamber of the eye. The cutting tip is ultrasonically vibrated along its longitudinal axis within the irrigating sleeve by the crystal-driven ultrasonic horn, thereby emulsifying upon contact the selected tissue in situ. The hollow bore of the cutting tip communicates with the bore in the horn that in turn communicates with the aspiration line from the handpiece to the console. A reduced pressure or vacuum source in the console draws or aspirates the emulsified tissue from the eye through the open end of the cutting tip, the bore of the cutting tip, the horn bore, and the aspiration line and into a collection device. The aspiration of emulsified tissue is aided by a saline flushing solution or irrigant that is injected into the surgical site through the small annular gap between the inside surface of the irrigating sleeve and the outside surface of the cutting tip.
There have been prior attempts to combine ultrasonic longitudinal motion of the cutting tip with rotational motion of the tip, see U.S. Pat. No. 5,222,959 (Anis), U.S. Pat. No. 5,722,945 (Anis, et al.) and U.S. Pat. No. 4,504,264 (Kelman), the entire contents of which are incorporated herein by reference. These prior attempts have used electric motors to provide the rotation of the tip which require O-ring or other seals that can fail in addition to the added complexity and possible failure of the motors.
There have also been prior attempts to generate both longitudinal and torsional motion without the use of electric motors. For example, in U.S. Pat. Nos. 6,028,387, 6,077,285 and 6,402,769 (Boukhny), one of the inventors of the current invention, describes a handpiece having two pairs of piezoelectric crystals are used. One pair is polarized to product longitudinal motion. The other pair is polarized to produce torsional motion. Two separate drive signals are used to drive the two pairs of crystals. In actual practice, making a handpiece using two pairs of crystals resonate in both longitudinal and torsional directions is difficult to achieve. One possible solution, also described by one of the current inventors, is described in U.S. Patent Publication No. US 2001/0011176 A1 (Boukhny). This reference discloses a handpiece have a single set of piezoelectric crystals that produces longitudinal motion, and a series of diagonal slits on the handpiece horn or tip that produce torsional motion when the horn or tip is driven at the resonate frequency of the piezoelectric crystals. Again, in practice, the resonate frequency of the piezoelectric crystals and the tip or horn did not coincide, so simultaneous longitudinal and torsional motion was difficult to achieve.
When the tip becomes occluded or clogged with emulsified tissue, the aspiration flow can be reduced or eliminated, allowing the tip to heat up, thereby reducing cooling and resulting in temperature increase, which may burn the tissue at the incision. In addition, during occlusion, a larger vacuum can build up in the aspiration tubing so that when the occlusion eventually breaks, a larger amount of fluid can be quickly suctioned from the eye, possibly resulting in the globe collapsing or other damage to the eye.
Known devices have used sensors that detect large rises in aspiration vacuum, and detect occlusions based a particular pre-determined vacuum level. Based on this sensed occlusion, power to the handpiece may be reduced and/or irrigation and aspiration flows can be increased. See U.S. Pat. Nos. 5,591,127, 5,700,240 and 5,766,146 (Barwick, Jr., et al.), the entire contents of which are incorporated herein by reference. These devices, however, use a fixed aspiration vacuum level to trigger a response from the system. This fixed level is a threshold value based upon a fixed percentage of the selected upper vacuum limit. The use and effectiveness of such systems, however, are limited since they do not respond until that preset vacuum level is reached. In addition, some surgical techniques require the plugging or occlusion of the tip, and the occurrence of an occlusion does not necessarily indicate that the tip and/or wound is getting heated sufficiently to create a concern or a thermal injury or bum at the wound site.
U.S. Pat. No. 4,827,911 (Broadwin, et al.) and U.S. Pat. No. 6,780,165 B2 (Kadziauskas, et al.) suggests that the risk of a thermal injury can be reduced by delivering the ultrasound energy in pulses of very short duration follow by a period wherein no energy is delivered to the tip. Such short pulses can help reduce the amount of energy entering the eye, but as the pulses get shorter, there is less time for the feedback loop to establish the optimum frequency. Current ultrasound handpiece tuning systems use a feedback loop to monitor the operation of the handpiece and continually tune the handpiece to ensure that the stock of the tip remains constant under all loading conditions. See U.S. Pat. No. 5,431,664 (Ureche, et al.). Such feedback loops typically take on the order of 3-5 milliseconds to cycle and automatically adjust the operating parameters of the handpiece. As a result, with current systems, ultrasonic power pulses of less than 5 milliseconds have limited ability to establish the optimum frequency, but in general, improvements to the tuning algorithm can be achieved for pulse durations of less than 20 milliseconds.
Accordingly, a need continues to exist for a reliable ultrasonic handpiece that is capable of delivering ultrasound pulses of less than 5 milliseconds while remaining in tune.